The present invention concerns a non-heat treated steel for soft nitriding, especially, the non-heat treated steel for soft nitriding having good fatigue strength and good straightening ability by bending. The steel is a useful material for manufacturing machine parts. The term xe2x80x9cnon-heat treated steelxe2x80x9d here means a steel for which normalization after hot forging is not necessary.
Machine parts such as gears, shafts, crankshafts and connecting rods have been manufactured with a carbon steel for machine structure (JIS S48 steel) by hot forging, normalizing and machining followed by soft-nitriding for the purpose of improving anti-seizure property, abrasion resistance and fatigue strength, and finally, finishing processing. Long materials as well as crankshafts, for which even slight bend is a problem, are subjected to straightening by bending.
In the conventional process, as noted above, normalizing is carried out after hot forging in order to regulate hardness, to make the crystal structure coarsened during the hot forging fine and uniform, and to improve soft-nitridability and straightening ability by bending. From the view to save costs and energy, however, there has been demand for non-heat treated steel for soft-nitriding.
Forged parts manufactured with such a non-heat treated steel by soft-nitriding is, as mentioned above, disclosed in Japanese Patent Disclosure No. 2000-8141. The non-heat treated steel comprises: by weight, C: 0.02-0.30%, Mn: 1.0-2.0%, P: up to 0.10%, Cr: 0-0.15%, s-Al: 0-0.01%, Ti: up to 0.02%, N: 0.010-0.030%, V: 0-0.02%, and optionally, one or more of S: 0.04-0.10%, Ca: 0.0003-0.0030% and Pb: 0.05-0.20%, the balance of Fe and inevitable impurities.
The non-heat treated steel for soft-nitriding to be material for the forged parts disclosed in the above Japanese Patent Disclosure contains a small amount of Ti in order to suppress austenite crystal growth during hot forging and to make the pearlite crystals occurring after cooling fine so as to minimize the initial cracks which may occur at the straightening by bending. The mechanism is that, even if cracks occur in pearlite crystals, propagation of the cracks is prevented by ferrite crystals which are distributed throughout the structure. However, it is difficult to make the pearlitic crystals uniformly small, and if the cracks occur in large pearlitic crystals, propagation of the cracks is easy, and thus, allowance for the bend-rectification is small.
The object of the present invention is to provide such a non-heat treated steel for soft nitriding, for which normalization after forging can be eliminated, that the steel can be processed to the parts which exhibit allowance for straightening by bending and fatigue strength larger than those of parts made with JIS S48C steel by forging, normalizing and soft nitriding.
This object is achieved by the non-heat treated steel according to the present invention.